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Modulation Analysis
R&S
®
Cable Rider ZPH
278
User Manual 1321.0950.02 ─ 07
Example: AM modulation with a modulating frequency of 1kHz
1. Calculate the demodulation bandwidth you require.
Demodulation Bandwidth
= 2 x 1 kHz
2. Select the minimum demodulation bandwidth from table
Table 12-2
.
As a result, the demodulation bandwidth of "3000" is selected from the table. This
shows that a minimum measurement time of 1.31 seconds is required to capture
3125 of sample rate.
3. Select the appropriate audio lowpass filter from
Table 12-3
.
4. Depending on the sample rate and modulating frequency range, "DBW/1" is
selected as the audio lowpass filter.
5. If a faster measurement time is desired, e.g. 6 ms, the audio lowpass, "DBW/10"
should be selected to fulfill the requirement on the modulating frequency.
Table 12-2: Relationship on Demodulation Bandwidth, Sample Rate and Measurement Time
Demodulation Bandwidth <=
(kHz), "Manual DBW"
Output Sample Rate
Measurement Time (ms), "MST"
2000000
2500000
1
989000
1250000
3
650000
964000
4
500000
625000
6
300000
391000
10
200000
250000
16
100000
125000
32
50000
62500
65
30000
36250
112
20000
25000
163
10000
12500
327
5000
6250
655
3000
3125
1310
Table 12-3: Selection of Audio Lowpass Filter
Selection of Audio
Lowpass, DPD (Digi-
tal Predistortion) fil-
ter
Output Sample Rate
Modulating Frequency (Hz)
Maximum Frequency
Minimum Frequency
"DBW/1"
2500000
1000000
6100
1250000
500000
3050
964000
325000
2350
625000
250000
1520
Analog Demodulation